The Aedes Mosquito: Carrier of More Than Just Zika

As Zika continues to spread, doctors and scientists have officially concluded that a causal relationship exists between pregnant women infected with Zika and microcephaly appearing in newborns (in addition to other brain anomalies).[1] Belize, Saint Lucia, and Vietnam are the latest countries to experience their first locally acquired cases. Meanwhile, Panama has recently announced its first case of microcephaly potentially linked to Zika, and Brazil is continuing to experience large-scale transmission of the virus, with a significant increase in cases of newborns suffering from microcephaly. And Colombia is experiencing an uptick in Zika cases, with the rainy season (i.e., more mosquitoes) just around the corner.

Spread of Zika

Image source: WHO

But Zika is not the only mosquito-borne virus of global concern. In this article, we trace the epidemiology (spread) of two similar arboviruses—dengue, and chikungunya—and the mosquitoes that primarily transmit them. This overview excludes other mosquito-borne conditions like malaria (214 million cases worldwide in 2015) and yellow fever (estimated 30,000 deaths every year). However, understanding where Zika comes from, which mosquitoes carry the virus, and how it is similar to (and different from) other arboviruses can help you recognize the health implications for your own situation as well as those for societies located in areas affected by these viruses.

Zika: What is it, and what’s the big deal?

Zika is an arbovirus, which is a class of viruses transmitted via arthropods (the prefix “arbo” is short for arthropod-borne). Arthropods are a family of invertebrates that include ticks, mosquitoes, spiders, and crustaceans. Not all arthropods can act as competent vectors (carriers) for Zika, however. In the case of Zika, the Aedes aegypti mosquito, which inhabits tropical and subtropical climates, is the primary vector of the disease at this point in time (more on this later). People become infected with Zika when they are bitten by a mosquito that is carrying the virus. Other potential methods of transmission exist, e.g., sexual intercourse, but mosquito bites by infected insects are by far the most common method of transmission.

The symptoms of Zika are mild, if they appear at all. In fact, roughly 80% of individuals infected with Zika virus do not notice any symptoms. And those who do develop symptoms typically experience only mild ones like fever, skin rashes, conjunctivitis (red eyes), muscle and joint pain, and general malaise. However, the medical community has now definitively determined a causal relationship between Zika and microcephaly (as well as other brain anomalies) in offspring. The medical community is concerned that Zika may also be linked to Guillain-Barré syndrome (GBS). Brazil’s recent explosion in Zika cases and reported cases of microcephaly has led a number of governments to issue travel warnings advising pregnant women to refrain from travelling to areas experiencing Zika outbreaks. In addition, there is growing concern and evidence that Zika may cause other spinal, brain, and fetal disorders in addition to GBS and microcephaly.

Are there other viruses like Zika?

Zika is only one member of the arbovirus family. There are a number of other viruses that belong to the same family, as well as a number of other conditions that are similar but not classified as arboviruses. Dengue virus is the most common arbovirus worldwide, with an estimated 40% of the world’s population living in areas with dengue transmission.[2] The symptoms of dengue are similar to those of Zika, but much more severe. Roughly 1 in 10 cases of severe dengue causes death (equating to roughly 1 in 2,000 total cases of dengue).[3]

Chikungunya virus is another common arbovirus. It only began to appear in the Americas in December 2013 but has since resulted in over 1.8 million suspected cases in Latin America, the Caribbean, and the United States.[4] Symptoms are again similar to those of dengue and Zika. Although researchers are developing treatments, dengue, chikungunya, and Zika currently cannot be prevented through the use of vaccines or prophylactic remedies. The only way to protect yourself is to prevent mosquito bites.

There are a number of other arboviruses, some of which you may be familiar with already: yellow fever, West Nile, various types of encephalitis, and others. In addition, other mosquito-borne conditions exist that result in similar symptoms and complications but are not considered arboviruses. The most widespread of these is malaria, which is a protozoan transmitted by the Anopheles mosquito. In the year 2015 alone, there were 214 million cases of malaria, and 438,000 deaths associated with the illness.[5] Nearly half the world’s population (3.2 billion people) is at risk. It is important to understand that many populations around the world, as well as travellers, are exposed to arboviruses and other similar illnesses, of which Zika is just one.

Primary vector – the Aedes mosquito

Chikungunya

Dengue

Zika

Aedes aegypti

Yes

Yes

Yes

Aedes albopictus

Yes

Rarely

Potentially

Aedes aegypti

The Aedes aegypti mosquito, commonly called the yellow fever mosquito, enjoys tropical and subtropical climates around the world. Originating in Africa, the Aedes aegypti is thought to have migrated to the Americas hundreds of years ago following the arrival of Europeans.[6] Some experts have found that most populations of Aedes aegypti in South America are genetically similar to those in Southeast Asia, leading to a belief that the Asian and Australian Aedes aegypti populations migrated across the Pacific from the Americas.[7][8]Aedes aegypti inhabits urban areas with or without vegetation, laying its eggs both indoors and outdoors.[9] It is a sneaky biter, with peak activity occurring around sunrise and sunset (including post-sunrise and pre-sunset).[10]

Image source: eLife 2015

Aedes albopictus

Aedes albopictus, commonly called the Asian tiger mosquito, inhabits a wider range of climates than its cousin Aedes aegypti. Aedes albopictus has shown the ability to adapt to tropical and subtropical climates, as well as cooler, drier, more temperate climates.[11] As a result, the Aedes albopictus mosquito has populations as far north as the Great Lakes in North America, and northern Italy and Slovenia in Europe. Native to East Asia, Aedes albopictus is the most invasive mosquito in the world and is now considered an invasive species on a number of continents. It arrived in the Americas much more recently than its cousin did, potentially being transported along with shipments of used tires from Asia. Aedes albopictus is mostly an outdoor mosquito associated with thickets and arboreal vegetation.[12] It is an aggressive daytime biter, with peak activity occurring around sunrise and sunset (including post-sunrise and pre-sunset).[13]

Image source: eLife 2015

Historical epidemiology

Dengue

Image source: CDC

Image source: CDC

Image source: CDC

There are four strains of dengue virus: DENV1, DENV2, DENV3, and DENV4. Although the virus has likely been circulating for hundreds of years, with reports of dengue-like cases dating back to the 19th century, the virus was first isolated in Japan (1943) and then in French Polynesia (1943) and Hawaii (1944).[14] A second strain (DENV2) was first reported in Papua New Guinea (1944). At that time, global travel associated with the Second World War is believed to have been a primary factor in the transmission of the virus across regions. Epidemics began to occur from India to the Philippines.

Moving into the second half of the 20th century, the transmission pattern of dengue closely followed that of the Aedes aegypti mosquito, with migration across oceans likely occurring as a result of increased urbanization and international movement of people and goods. A large-scale Aedes aegypti eradication program was undertaken in the Americas in the 1960s into the early 1970s, which effectively reduced populations of the mosquito as well as associated dengue epidemics.[15] However, the collapse of the eradication campaign in the 1970s led to a rapid return of both Aedes aegypti and dengue.

Dengue is now endemic to many parts of the tropics and subtropics. Consequently, it occurs every year, typically during a season when Aedes aegypti mosquito populations are high, like wet seasons. Most cases of dengue are asymptomatic, but when symptoms are present, they can take the form of mild dengue fever (DF) or more severe forms called dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS).[16] Dengue is the most common arboviral disease worldwide, with an estimated 40% of the global population living in areas with dengue transmission. The World Health Organization estimates that 50–100 million infections occur every year, 500,000 of which are the severe DHF, and 22,000 of which lead to fatality (mostly in children).[17]

Chikungunya

Image source: CDC

Image Source: PAHO

Chikungunya virus was first detected in an epidemic along the border of Mozambique and Tanzania (1952–1953). An outbreak in Bangkok in 1958 was the first known incidence of chikungunya in Asia. Outbreaks continued to spread in India, Sri Lanka, Cambodia, Vietnam, Myanmar, Laos, and Indonesia up until the mid-1980s, when there was a decrease in chikungunya outbreaks.

The virus re-emerged at the turn of the century. It appeared in Malaysia (1998–1999), produced a large outbreak in the Democratic Republic of the Congo (1999–2000), and emerged in Kenya (2004). From Kenya, the virus quickly spread eastward in the mid-2000s, appearing on western Indian Ocean islands and then in India, Thailand, and Indonesia.[18] On the western Indian Ocean island of Réunion, there was a large outbreak that affected an estimated 266,000 people (34.5% of the island’s population). The Réunion outbreak was medically significant because the chikungunya virus had genetically mutated, allowing the Aedes albopictus mosquito to act as an efficient vector (previously, the Aedes aegypti had been the primary vector of chikungunya).[19] Soon after, an epidemic struck a number of states in India that resulted in an estimated 1.25 million cases of chikungunya.

From there, chikungunya was reported in Europe for the first time, when over 200 people were infected in northeastern Italy in 2007. Outbreaks continued to occur across Southeast Asia, and the first local transmission was reported in the Americas in the French Caribbean department of St. Martin (Dec. 2013). Over the next few months, chikungunya spread to multiple Caribbean islands, with June 2014 estimates reporting that more than 165,000 people in the Americas had already been affected by local transmission of the virus in just a half a year. The virus continued to spread, with locally acquired cases occurring in France later that year.

Chikungunya is now present in many parts of the tropics, subtropics, and more temperate climates due to the ability of both Aedes aegypti and Aedes albopictus to act as efficient vectors, although the former remains the primary carrier. As with dengue, the epidemics generally occur during wet or rainy seasons, although outbreaks in Africa have occurred after droughts (with open water containers acting as breeding sites for mosquitoes).[20] Since 2005, India, Indonesia, the Maldives, Myanmar, and Thailand have seen 1.9 million cases of chikungunya.[21] Likewise, since December 2013, when the virus first appeared in the Americas, there have been over 1.8 million suspected cases in Latin America, the Caribbean, and the United States.[22] While the virus does not often result in death (although fatalities are not uncommon), severe joint pain can last for months or years.

Zika

Image source: WHO

Image source: WHO

Zika virus was first isolated in Uganda (1947) during an experiment meant to collect data on yellow fever in Rhesus monkeys. Although the virus had likely been present in African monkeys (or some other host) for a long period of time, this was the first time it had been isolated. Shortly afterwards, Zika antibodies were found to be naturally occurring in populations of humans in Uganda and Tanzania (1952), and the virus was isolated in a human for the first time in Nigeria (1954).[23] From the late 1960s to the early 1980s, Zika expanded in geographical distribution, migrating to equatorial Asia, including Pakistan, India, Malaysia, and Indonesia. However, there were no documented outbreaks of the disease until 2007.

An outbreak of Zika on Yap Island in the Southwestern Pacific (2007) brought the virus back to the attention of the international medical community. The outbreak had resulted in an estimated 73% of the island’s population over three years of age being infected with Zika.[24] A few years later, a number of other Pacific islands experienced outbreaks. French Polynesia saw a large outbreak (2013), with 28,000 people (11% of the population) estimated to have been sent into medical care. Zika was first detected in the Americas a year later (2014), and Brazil experienced an outbreak in the northeastern region of the country beginning in March 2015. This represented the first time the virus had been reported on the South American mainland.

As of April 2016, Zika virus was present in 35 countries in the Americas, 17 territories in Oceania/the Pacific Islands, and Cabo Verde in Africa, with Belize, Saint Lucia, and Vietnam being the most recent additions to the list of countries with locally acquired cases. Brazil has estimated that there have been as many as 1.5 million cases within its borders already, while Colombia is the second-most-affected country, with 64,839 confirmed and suspected cases as of early April 2016.[25] Colombia’s rainy season will begin soon, and the mosquito population will explode. In other words, the number of Zika cases in the country will likely skyrocket. The Colombian government fears the country will see over 600,000 cases by the end of the year. The World Health Organization has warned that Zika may spread through the Americas, with the only exceptions being Canada and Chile, officially calling the situation a Public Health Emergency of International Concern (PHEIC). This puts it in the same class as swine flu, polio, and Ebola over the past several years.

Conclusion

Zika is a global health issue. It is already spreading around the world, affecting large numbers of people and following patterns of spread similar to those of other arboviruses. If the Aedes albopictus mosquito is found to be a competent vector of Zika, which some medical professionals suspect is likely, then the virus may also spread further into more temperate climates.[26] Fortunately, roughly 80% of Zika cases are symptom-free. The danger lies with the consequences for pregnant women living in or travelling to affected areas. Pregnant women infected with Zika, even if they do not have any symptoms, are at an increased risk of having children with microcephaly or other disorders. With such large numbers of cases occurring, the result could be a generational burden on society in areas that are heavily affected by Zika, particularly where social institutions are nonexistent or are ill equipped to deal with these types of situations.

Dengue and chikungunya, which can cause long-term health complications and even death, are already present in many areas of the world. Forty per cent of the world’s population live in areas with dengue transmission, with between 20,000 and 25,000 fatalities occurring every year, primarily in children. Millions of people have been infected with chikungunya over the past several years, and the virus has only recently arrived in the Americas. Malaria, transmitted by the Anopheles mosquito, is prevalent in areas where 3.2 billion people live, causing 214 million cases and 438,000 deaths in 2015 alone.

Zika is a global health concern, even more so now that it has been causally linked to microcephaly and other serious disorders. But there are other similar viruses and afflictions that have already affected millions, even hundreds of millions, of people. The importance of being adequately informed cannot be stressed enough, particularly since currently there are no vaccines or prophylactic treatments for these viruses.

On a societal level, preventing the transmission of mosquito-borne illnesses can include releasing genetically modified mosquitoes into the wild or informing populations about the need to reduce local mosquito breeding sites and to practise personal prevention measures. On a personal level, you can protect yourself from mosquitoes by using insect repellent containing DEET (safe to use if pregnant), wearing light-coloured long-sleeved shirts and pants, using mosquito netting, and staying in places with air conditioning or window and door screens.

We urge you to stay informed, whether for your own health, your loved ones’ wellbeing, your employees’ safety, or a general global human interest. There are reliable resources that outline which health conditions are prevalent in every country and territory around the world. Staying up to date with factual information about your own country of residence, neighbouring countries, and areas you plan on visiting is a simple and effective way to empower yourself and mitigate the risk of contracting illnesses.

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